WASHINGTON — American counterterrorism officials are increasingly concerned that the most dangerous regional arm of Al Qaeda is trying to produce the lethal poison ricin, to be packed around small explosives for attacks against the United States.

For more than a year, according to classified intelligence reports, Al Qaeda’s affiliate in Yemen has been making efforts to acquire large quantities of castor beans, which are required to produce ricin, a white, powdery toxin that is so deadly that just a speck can kill if it is inhaled or reaches the bloodstream.

Intelligence officials say they have collected evidence that Qaeda operatives are trying to move castor beans and processing agents to a hideaway in Shabwa Province, in one of Yemen’s rugged tribal areas controlled by insurgents. The officials say the evidence points to efforts to secretly concoct batches of the poison, pack them around small explosives, and then try to explode them in contained spaces, like a shopping mall, an airport or a subway station.

President Obama and his top national security aides were first briefed on the threat last year and have received periodic updates since then, top aides said. Senior American officials say there is no indication that a ricin attack is imminent, and some experts say the Qaeda affiliate is still struggling with how to deploy ricin as an effective weapon.

These officials also note that ricin’s utility as a weapon is limited because the substance loses its potency in dry, sunny conditions, and unlike many nerve agents, it is not easily absorbed through the skin. Yemen is a hot, dry country, posing an additional challenge to militants trying to produce ricin there.

But senior American officials say they are tracking the possibility of a threat very closely, given the Yemeni affiliate’s proven ability to devise plots, including some thwarted only at the last minute: a bomb sewn into the underwear of a Nigerian man aboard a commercial jetliner to Detroit in December 2009, and printer cartridges packed with powerful explosives in cargo bound for Chicago 10 months later.

“The potential threat of weapons of mass destruction, likely in a simpler form than what people might imagine but still a form that would have a significant psychological impact, from Al Qaeda in the Arabian Peninsula in Yemen, is very, very real,” Michael E. Leiter, who retired recently as director of the National Counterterrorism Center, said at a security conference last month. “It’s not hard to develop ricin.”

Photo

Anwar al-Awlaki, an American in the Qaeda branch.Credit
Muhammad ud-Deen, via Associated Press

A range of administration officials have stated that the threat of a major attack from Al Qaeda’s main leadership in Pakistan has waned after Osama bin Laden’s death in May, on top of the Central Intelligence Agency’s increasing drone assaults on Qaeda targets in Pakistan’s tribal areas over the past three years.

But the continuing concern over a ricin plot underscores the menace that regional Qaeda affiliates, especially Al Qaeda in the Arabian Peninsula, now pose to the United States and American interests overseas.

“That line of threat has never abated,” said a senior American official, who referred to the terrorist group by its initials. “That’s been taken seriously by this government. What we know about A.Q.A.P. is that they do what they say.”

Al Qaeda’s arm in Yemen has openly discussed deploying ricin and other deadly poisons against the United States. “Brothers with less experience in the fields of microbiology or chemistry, as long as they possess basic scientific knowledge, would be able to develop other poisons such as ricin or cyanide,” the organization posted to its online English-language journal, Inspire, last fall, in an article titled “Tips for Our Brothers in the United States of America.”

Senior administration officials say ricin is among the threats focused on by a secret government task force created after the printer-cartridge plot. The task force is working closely with Saudi intelligence officials and the remnants of Yemen’s intelligence agencies, and it is using information gleaned from the shipboard interrogation of a Somali terrorist leader with ties to the Yemeni branch of Al Qaeda, who was captured by Navy Seal commandos in April.

The intelligence reports indicating ricin plots by Al Qaeda’s Yemeni affiliate were first uncovered during reporting for a book, “Counterstrike: The Untold Story of America’s Secret Campaign Against Al Qaeda.” It will be published next week by Times Books, an imprint of Henry Holt & Company.

American officials now say that Al Qaeda’s most direct threat to the United States comes from the Yemeni affiliate. These officials have also expressed growing alarm at the way the affiliate is capitalizing on the virtual collapse of Yemen’s government to widen its area of control inside the country, and is strengthening its operational ties to the Shabab, the Islamic militancy in Somalia, to exploit the chaos in both countries.

“It continues to demonstrate its growing ambitions and strong desire to carry out attacks outside its region,” Daniel Benjamin, the State Department’s counterterrorism coordinator, said in a speech last month, referring to Al Qaeda’s Yemeni branch.

The affiliate has also become a magnet for terrorists fleeing the increasing pressure from drone strikes in Pakistan, and is recruiting specialists in bomb-making and other skills. “These guys have got some notoriety,” said a senior United States official who follows Al Qaeda and its affiliates closely. “They have a natural, charismatic attraction value for people who want to be jihadists and plot against the West.”

“A.Q.A.P.’s senior leaders are a lot like an organization that’s largely a brain that exists on its own and has to recruit its arms and legs to actually execute things,” the official continued.

Largely because of the Americans in the Yemeni affiliate’s top leadership, including Anwar al-Awlaki, a cleric born in New Mexico who is in hiding in Yemen, American counterterrorism and intelligence officials fear the affiliate’s innovative agility. “The fastest-learning enemy we have is A.Q.A.P.,” said the senior United States official.

An error has occurred. Please try again later.

You are already subscribed to this email.

In recent months, as the Yemeni government has become nearly paralyzed, the Obama administration has stepped up pressure on the Qaeda affiliate there. It has escalated a campaign of airstrikes carried out by the Pentagon’s Joint Special Operations Command with the C.I.A.’s help. The C.I.A. is building a base in the region to serve as a hub for future operations in Yemen.

The Pentagon’s air campaign in Yemen was renewed in May after a nearly yearlong hiatus; since then the military has carried out at least four airstrikes in the country.

The ricin plots believed to be emanating from Yemen are the latest example of terrorists’ desire to obtain and deploy unconventional weapons in attacks. In 1995, the Aum Shinrikyo cult released sarin nerve gas on underground trains in Tokyo, killing 12 people and injuring more than 5,000, and nearly paralyzing one of the world’s leading economies for weeks.

In 2003, British and French operatives broke up suspected Qaeda cells that possessed components and manuals for making ricin bombs and maps of the London subway system.

A ricin-dispersing bomb detonated in a major subway system or in a mall or at a major airport would not result in mass destruction on the scale of the attacks on Sept. 11, 2001, counterterrorism specialists said. But it could inflict disproportionate psychological terror on big-city transportation systems. “Is it going to kill many people? No,” said Mr. Leiter, the former counterterrorism official. “Is it going to be a big news story and is it going to scare some people? Yes.”

Months after the initial ricin intelligence reports surfaced last year, Saudi intelligence officials revealed a twist to the ricin plot: Qaeda operatives were trying to place the toxin in bottles of perfume, especially a popular local fragrance made of the resin of agarwood, and send those bottles as gifts to assassinate government officials and law enforcement and military officers. There is no indication that Al Qaeda ever succeeded with this approach, intelligence officials said.

A version of this article appears in print on August 13, 2011, on Page A1 of the New York edition with the headline: Qaeda Trying to Harness Toxin For Bombs, U.S. Officials Fear. Order Reprints|Today's Paper|Subscribe